Our current emphasis in the study of play is investigation of the motivational or social conditions promoting the appearance of playful behavior in the squirrel monkey. Developmental and functional analyses of human play have been the subject of extensive clinical interest, particularly as a symptom and early predictor of psychopathology. Comparable studies of play in animals have been few, although it is commonly assumed that play in animals is an indicator of good mental as well as physical health. Play behavior is ubiquitous in young mammals and is usually the chief means of social interaction between youngsters. Nonetheless, play appears to be quite sensitive to stressors, dropping out of the repertoire, or being greatly curtailed, in response to nutritional or environmental stress, and in some cases to emotional stress, e.g. loss of the mother. In captive animals, particularly juveniles, playfulness is thought to be a sign of general well-being. The present experiments are designed to investigate a possible relationship between a physiological measure of stress (plasma cortisol level) and a behavioral assessment of play, using as subjects a population of young squirrel monkeys, genus Saimiri. Juvenile squirrel monkeys exhibit an extensive repertoire of play behaviors and, independently, their physiological and endocrine responses have been well studied, providing a particularly favorable opportunity to correlate physiology and behavior. The first hypothesis being investigated is that an inverse relationship exists between an individual monkey's plasma cortisol level (a measure of stress) and his or her level of play. Our results so far indicate that the predicted negative relationship is true; plasma cortisol and amount of time spent in play (measured on a rating scale) showed a significant negative correlation both for the baseline condition and over all conditions in the study. The experimental paradigm attempted to manipulate stress by altering an individual's social environment; re-pairing animals with new partners increased their cortisol levels reliably, particularly for animals with low baseline levels. Play was also disrupted by re-pairing, as expected. Establishing a correlation between stress and play (essentially, providing a sound basis for the conventional wisdom that holds that they are incompatible) has practical as well as theoretical significance. As a result of recent and pending legislation, investigators working with captive primates must assure the well-being of the animals in their care. Yet few assessment measures exist. Measuring (or simply observing) play may be an expedient, non-invasive and supportable indicator of psychological well-being for young animals, particularly useful in light of new mandates requiring pair housing of primates wherever possible.